Evaporative heat exchange apparatus with finned elliptical tube coil assembly

ABSTRACT

An improved finned coil tube assembly enhances evaporative heat exchanger performance, and includes tubes, preferably serpentine tubes, in the coil assembly. The tubes have a generally elliptical cross-section with external fins formed on an outer surface of the tubes. The fins are spaced substantially 1.5 to substantially 3.5 fins per inch (2.54 cm) along the longitudinal axis of the tubes, extend substantially 23.8% to substantially 36% of the nominal tube outside diameter in height from the tubes outer surface and have a thickness of substantially 0.007 inch (0.018 cm) to substantially 0.020 inch (0.051 cm). The tubes have a center-to-center spacing generally horizontally and normal to the longitudinal axis of the tubes of substantially 109% to substantially 125% of the nominal tube outside diameter, and a generally vertical center-to-center spacing of substantially 100% to about 131% of the nominal tube outside diameter.

BACKGROUND OF THE INVENTION

The present invention relates to improvements in tubes in a coilassembly for use in an evaporative heat exchange apparatus in which thecoil assembly is to be mounted in a duct or plenum of the apparatus inwhich external heat exchange fluids, typically a liquid, usually water,and a gas, usually air, flow externally through the coil assembly tocool an internal heat transfer fluid passing internally through thetubes of the coil assembly. The improvements concern the use of tubes orsegments of the tubes having a generally elliptical cross-section, incombination with tube orientation, arrangement and spacing, and finspacing, height and thickness, all of which must be carefully balanced,to provide increased heat transfer coefficients with an unexpectedrelatively low air pressure drop that produces high air volume thattogether produces very high heat exchange capacity.

Preferably, though not exclusively, the finned tube coil assembly of thepresent invention using tubes that have finned segments with generallyelliptical cross-sections, is most effectively mounted in a counterflowevaporative heat exchanger so that water flows downwardly and externallythrough the coil assembly while air travels upwardly and externallythrough the coil assembly. The coil assembly of the present inventioncan be used also in a parallel flow evaporative heat exchanger in whichthe air travels in the same direction over the coil assembly as thewater, as well as in a crossflow evaporative heat exchanger, where airtravels over the coil in a direction transverse to the flow of thewater. The evaporation of the water cools the coil assembly and theinternal heat transfer fluid inside the tubes forming the coil assembly.

The tubes may be used in any type of evaporative heat exchange coilassembly made of an array of several, and preferably, many tubes thatcan have a variety of arrangements. The tubes are preferably arranged ingenerally horizontal rows extending across the flow path of the air andwater which flow externally through the coil assembly, whether the airand water are in counterflow, parallel flow or crossflow pathways. Theends of the tubes may be connected to manifold or headers forappropriate distribution of the internal heat transfer fluid. Theinternal heat transfer fluid may be a heating fluid, a cooling fluid ora processing fluid used in various types of industrial processes, wherethe temperature of the internal heat transfer fluid needs to bemodified, typically but not exclusively by cooling, and often but notexclusively by condensing, as a result of the heat transfer through thewalls of the tubes by the external heat exchange fluids.

Typically, evaporative heat exchange apparatus use a number ofserpentine tubes for the coil assemblies, and such serpentine tubes areoften the preferred type of tubes used due to the ease of manufacture ofeffective coil assemblies from such tubes. While other types of tubes ofthe present invention useful for the evaporative heat exchange apparatusof the present invention, the tubes and coil assemblies of the presentinvention will primarily be described, without limitation, with respectto the preferred serpentine tubes. The following background informationis provided to better understand the relationship of the tube and coilassembly components using serpentine tubes. Each serpentine tubecomprises a plurality of two different types of portions, “segments” and“return bends.” The segments are generally straight tube portions whichare connected by the return bends, which are the curved portions,sometimes referred to as “bights,” to give each tube its serpentinestructure. In a preferred embodiment of the coil assembly of the presentinvention, the tubes, which may be generally straight in structure(referred to hereinafter as “straight tubes”), or the segments of eachof the serpentine tubes, are generally elliptical in cross-section andthe return bends can be any desired shape and are typically generallycircular, generally elliptical, generally kidney-shaped or some othershape in cross-section. The generally horizontal maximum dimension ofthe generally elliptical segments is usually equal to or smaller thanthe generally horizontal cross-sectional dimension of the return bends,especially if the return bends have a circular cross-section. Ifdesired, the return bends can have an elliptical cross-section, or akidney-shaped cross-section, but it is usually easier to make the returnbends with a circular cross-section. The segments of horizontallyadjacent serpentine tubes are spaced from each other by the largerhorizontal cross-section of the return bends when the return bends arein contact with each other, or may be spaced by vertically-orientedspacers between the return bends, depending on the designcharacteristics of the evaporative heat exchange apparatus in which thecoil assemblies are used.

In the coil assemblies, the straight tubes or the segments of theserpentine tubes are preferably arranged in generally horizontal rowsextending across the flow path of the air and water which flowexternally through the coil assembly, whether the air and water are incounterflow, parallel flow or crossflow pathways.

Evaporative heat exchangers using coil assemblies using serpentine tubeshaving segments with generally elliptical cross-sections are also known,for example as disclosed in U.S. Pat. Nos. 4,755,331 and 7,296,620, thedisclosures of which are hereby incorporated herein in their entireties,which are assigned to Evapco, Inc., the assignee of the presentinvention. These patents do not disclose or contemplate the use offinned tubes in the coil assembly in the evaporative heat exchangeenvironment.

Finned tubes used in coil assemblies of dry (non-evaporative) heatexchangers are known and are used in view of the greater surface areaprovided by the fins to dissipate heat by conduction when exposed to airflowing externally through the coil assembly of the dry heat exchanger.Generally, the fins in such dry heat exchangers do not materiallyadversely affect the flow of air through the coil assembly of the dryheat exchanger. Finned coils are also used extensively in coilassemblies of products like home refrigerators to dissipate the heat tothe ambient air.

Examples of coil assemblies for dry heat exchangers made using fins inthe form of sheets or plates having holes though which segments havinggenerally elliptical cross-sections pass are disclosed in Evapco, Inc.'sU.S. Pat. Nos. 5,425,414, 5,799,725, 6,889,759, and 7,475,719. However,such coil assemblies are not useful with evaporative heat exchangers,since the sheets or plates would adversely affect the mixing andturbulence of the air and water involved with evaporative heat exchangethat must pass externally through the coil assembly.

Evapco, Inc. and others have used finned tube coil assemblies inevaporative heat exchangers where the segments of the tubes in the coilassemblies have circular cross-sections that include fins extendingalong the length of the individual segments of the tubes. The segmentshaving circular cross-sections are relatively easy to provide with fins,such as by spirally wrapping the segments with strips of metal formingthe fins. These finned tubes have been used in evaporative heatexchangers, but in limited circumstances and with limited success.First, round tube coils with fins have been employed in heat exchangersto enhance dry cooling capacity in cold weather applications when notmuch capacity is needed and when using water as an external heatexchange liquid could result in freezing and other problems. Such useswere rather rare and were provided to deal with a problem, as opposed toa way to improve the primary function of evaporative cooling accordingto the present invention. Second, though round tube coils with fins havealso been employed to improve evaporative cooling, this has not beensuccessful. While the presence of the fins increases the heat transfercoefficient, in prior attempts the increases were offset because thefins also caused decreased air flow over the coil, thus resulting inlower performance.

The finned tube coil assembly of the present invention provides a numberof significant advantages. The combination of the shape of the tubes,the spacing of the tubes, the height of the fins, and the number of finsper inch have resulted in exceptional and unexpected increases inevaporative thermal performance. The geometry of the tubes and theirorientation and arrangement with a coil assembly play an essential partin the turbulent mixing of the air and water. The generally ellipticalcross-sectional shape of the segments provides the advantages of a largeamount of surface area of the tubes in a coil assembly, effective flowand heat transfer of process fluid internally within the tubes andenhanced external air and water flow characteristics. With the presentinvention, the surprising result of less resistance to the air and waterpassing externally through the coil assembly allows the use of higherair volume that provides additional thermal capacity compared to theprior art systems without adding any fan energy. The finned tubesprovide an enhanced surface area for conductive heat exchange with thetubes and aid in turbulent mixing of the air and water externallyflowing through the coil assembly, enhancing convective heat exchangebetween the air and the water. The finned tubes take up space that mayimpede the water and air flow and thereby would be expected to cause avery significant air side pressure drop, with the need for strongermotors for fans to move the air through the coil assembly in the heatexchanger. However, the finned tubes with generally ellipticalcross-sections having the characteristics of the present invention notonly provide a careful balance of enhanced coil assembly surface areafor conductive heat exchange with any fluid flowing within the interiorof the tubes and mixing and turbulence of the air and water for theconvective heat exchange but also provide a surprising reduction in theair side pressure drop through the coil assembly, while retaining a verylarge increase in external heat transfer coefficient.

The overall capacity of the coil assembly of the present invention andevaporative heat exchangers containing it are greatly improved atnominal, or in certain circumstances even reduced cost, compared to theincrease in capacity. For example, the cost per cooling ton may bereduced by, for instance, replacing a coil assembly using morenon-finned tubes with a coil assembly using fewer finned tubes of thepresent invention. Additionally, an evaporative heat exchanger of agiven size using non-finned tubes of the prior art could be replacedwith a smaller evaporative heat exchanger according to the presentinvention that achieves the same or better thermal performance.Moreover, using a coil assembly having the finned tubes of the presentinvention could significantly reduce required fan energy, and thereforeoverall power consumption, as compared to a non-finned coil assembly ofthe same size.

Various types of heat exchange apparatus are used in a variety ofindustries, from simple building air conditioning to industrialprocessing such as petroleum refining, power plant cooling, and otherindustries. Typically, in indirect heat exchange systems, a processfluid used in any of such or other applications is subject to heating orcooling by passing internally through a coil assembly made of heatconducting material, typically a metal, such as aluminum, copper,galvanized steel or stainless steel. Heat is transferred through thewalls of the heat conducting material of the coil assembly to theambient atmosphere, or in a heat exchange apparatus, to other heatexchange fluid, typically air and/or water flowing externally over thecoil assembly where heat is transferred, usually from hot processingfluid internally within the coil assembly to the cooling heat exchangefluid externally of the coil assembly, by which the internal processingfluid is cooled and the external heat exchange fluid is warmed.

In evaporative indirect heat exchange apparatus in which the finned tubecoil assembly of the present invention is used, heat is transferredusing indirect evaporative exchange, where there are three fluids: agas, typically air (accordingly, such gas will usually be referred toherein, without limitation, as “air”), a process fluid flowinginternally through a coil assembly of tubes, and an evaporative coolingliquid, typically water (accordingly, such external heat exchange orcooling liquid will usually be referred to herein, without limitation,as “water”), which is distributed over the exterior of the coil assemblythrough which the process fluid is flowing and which also contacts andmixes with the air or other gas flowing externally through the coilassembly. The process fluid first exchanges sensible heat with theevaporative liquid through indirect heat transfer between the tubes ofthe coil assembly, since it does not directly contact the evaporativeliquid, and then the air stream and the evaporative liquid exchange heatand mass when they contact each other, resulting in more evaporativecooling.

In other embodiments, direct evaporative heat exchange may be usedtogether with the indirect evaporative heat exchange involving thefinned tube coil assembly of the present invention, as explained in moredetail hereinafter, to provide enhanced capacity. In direct evaporativeheat exchange apparatus, air or other gas and water or other coolingliquid may be passed through direct heat transfer media, called wet deckfill, where the water or other cooling liquid is then distributed as athin film over the extended fill surface for maximum cooling efficiency.The air and water contact each other directly across the fill surface,whereupon a small portion of the distributed water is evaporated,resulting in direct evaporative cooling of the water, which is usuallycollected in a sump for recirculation over the wet deck fill and anycoil assembly used in the apparatus for indirect heat exchange.

Evaporative heat exchangers are commonly used to reject heat as coolersor condensers. Thus, the apparatus of the present invention may be usedas a cooler, where the process fluid is a single phase fluid, typicallyliquid, and often water, although it may be a non-condensable gas at thetemperatures and pressures at which the apparatus is operating. Theapparatus of the present invention may also be used as a condenser,where the process fluid is a two-phase or a multi-phase fluid thatincludes a condensable gas, such as ammonia or FREON® refrigerant orother refrigerant in a condenser system at the temperatures andpressures at which the apparatus is operating, typically as part of arefrigeration system where the process fluid is compressed and thenevaporated to provide the desired refrigeration. Where the apparatus isused as a condenser, the condensate is collected in one or morecondensate receivers or is transferred directly to the associatedrefrigeration equipment having an expansion valve or evaporator wherethe refrigeration cycle begins again.

The present invention uses a finned tube coil assembly where the claimedcombination of factors of tube shape, orientation, arrangement andspacing, and fin spacing, height and thickness, all of which must becarefully balanced, to provide increased heat transfer coefficients withan unexpected relatively low air pressure drop that produces high airvolume. The combination of increased heat transfer coefficients withhigh air volume produces very high heat exchange capacity.

DEFINITIONS

As used herein, the singular forms “a”, “an”, and “the” include pluralreferents, and plural forms include the singular referent unless thecontext clearly dictates otherwise.

Certain terminology is used in the following description for convenienceonly and is not limiting. Words designating direction such as “bottom,”“top,” “front,” “back,” “left,” “right,” “sides,” “up” and “down”designate directions in the drawings to which reference is made, but arenot limiting with respect to the orientation in which the invention andits components and apparatus may be used. The terminology includes thewords specifically mentioned above, derivatives thereof and words ofsimilar import.

As used herein, the term “about” with respect to any numerical value,means that the numerical value has some reasonable leeway and is notcritical to the function or operation of the component being describedor the system or subsystem with which the component is used, and willinclude values within plus or minus 5% of the stated value.

As used herein, the term “generally” or derivatives thereof with respectto any element or parameter means that the element has the basic shape,or the parameter has the same basic direction, orientation or the liketo the extent that the function of the element or parameter would not bematerially adversely affected by somewhat of a change in the element orparameter. By way of example and not limitation, the segments having a“generally elliptical cross-sectional shape” refers not only to across-section of a true mathematical ellipse, but also to ovalcross-sections or somewhat squared corner cross-sections, or the like,but not a circular cross-section or a rectangular cross-section.Similarly, an element that may be described as “generally normal” to or“generally parallel to” another element can be oriented a few degreesmore or less than exactly 90° with respect to “generally normal” and afew degrees more or less than exactly perfectly parallel or 0° withrespect to “generally parallel,” where such variations do not materiallyadversely affect the function of the apparatus.

As used herein, the term “substantially” with respect to any numericalvalue or description of any element or parameter means precisely thevalue or description of the element or parameter but within reasonableindustrial manufacturing tolerances that would not adversely affect thefunction of the element or parameter or apparatus containing it, butsuch that variations due to such reasonable industrial manufacturingtolerances are less than variations described as being “about” or“generally.” By way of example and not limitation, “fins having a heightextending from the outer surface of the segments a distance ofsubstantially 23.8% to substantially 36% of the nominal tube outsidediameter” would not allow variations that adversely affect performance,such that the fins would be too short or too tall to allow theevaporative heat exchanger to have the desired enhanced performance.

As used herein, the term “thickness” with respect to the thickness ofthe fins, refers to the thickness of the fins prior to treatment afterthe fins are applied to the tubes to make the finned tubes, such asgalvanizing the tubes or the coil assembly using the finned tubes, assuch treatment would likely affect the nominal thickness of the fins,the nominal fin height and the nominal spacing of the fins. Thus, all ofthe dimensions set forth herein are of the finned tubes prior to anylater treatment of the finned tubes themselves or of any coil assemblycontaining them.

As used herein, where specific dimensions are presented in inches andparenthetically in centimeters (cm), the dimensions in inches controls,as the centimeter dimensions were calculated based on the inchesdimensions by multiplying the inches dimensions by 2.54 cm per inch androunding the centimeter dimensions to no more than three decimal places.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to an improvement in an evaporative heatexchanger comprising a plenum having a generally vertical longitudinalaxis, a distributor for distributing an external heat exchange liquidinto the plenum, an air mover for causing air to flow in a directionthrough the plenum in a direction generally countercurrent to, generallyparallel to, or generally across the longitudinal axis of the plenum,and a coil assembly having a major plane and being mounted within theplenum such that the major plane is generally normal to the longitudinalaxis of the plenum and such that the external heat exchange liquid flowsexternally through the coil assembly in a generally vertical flowdirection, wherein the coil assembly comprises inlet and outletmanifolds and a plurality of tubes connecting the manifolds, the tubesextending in a direction generally horizontally and having alongitudinal axis and a generally elliptical cross-sectional shapehaving a major axis and a minor axis where the average of the major axislength and the minor axis length is a nominal tube outside diameter, thetubes being arranged in the coil assembly such that adjacent tubes aregenerally vertically spaced from each other within planes generallyparallel to the major plane, the adjacent tubes in the planes generallyparallel to the major plane being staggered and spaced with respect toeach other generally vertically to form a plurality of staggeredgenerally horizontal levels in which every other tube is aligned in thesame generally horizontal level generally parallel to the major plane,and wherein the tubes are spaced from each other generally horizontallyand generally normal to the longitudinal axis of the tube.

The improvement comprises the tubes having external fins formed on anouter surface of the tubes, wherein the fins have a spacing ofsubstantially 1.5 to substantially 3.5 fins per inch (2.54 cm) along thelongitudinal axis of the tubes, the fins having a height extending fromthe outer surface of the tubes a distance of substantially 23.8% tosubstantially 36% of the nominal tube outside diameter, the fins havinga thickness of substantially 0.007 inch (0.018 cm) to substantially0.020 inch (0.051 cm), the tubes having a center-to-center spacinggenerally horizontally and generally normal to the longitudinal axis ofthe tubes of substantially 100% to substantially 131% of the nominaltube outside diameter, and the horizontally adjacent tubes having agenerally vertical center-to-center spacing of substantially 110% tosubstantially 300% of the nominal tube outside diameter.

Preferably, the tubes are serpentine tubes having a plurality ofsegments and a plurality of return bends, the return bends beingoriented in generally vertical planes, the segments of each tubeconnecting the return bends of each tube and extending between thereturn bends in a direction generally horizontally, the segments havinga longitudinal axis and a generally elliptical cross-sectional shapehaving a major axis and a minor axis where the average of the major axislength and the minor axis length is a nominal tube outside diameter, thesegments being arranged in the coil assembly such that the segments ofadjacent tubes are generally vertically spaced from each other withinplanes generally parallel to the major plane, the segments of adjacenttubes in the planes generally parallel to the major plane beingstaggered and spaced with respect to each other generally vertically toform a plurality of staggered generally horizontal levels in which everyother segment is aligned in the same generally horizontal levelgenerally parallel to the major plane, and wherein the segments arespaced from each other generally horizontally and generally normal tothe longitudinal axis of the segment connected to the return bend.

Where the tubes are serpentine tubes, the improvement comprises thesegments having external fins formed on an outer surface of thesegments, wherein the fins have a spacing of substantially 1.5 tosubstantially 3.5 fins per inch (2.54 cm) along the longitudinal axis ofthe segments, the fins having a height extending from the outer surfaceof the segments a distance of substantially 23.8% to substantially 36%of the nominal tube outside diameter, the fins having a thickness ofsubstantially 0.007 inch (0.018 cm) to substantially 0.020 inch (0.051cm)%, the segments having a center-to-center spacing generallyhorizontally and generally normal to the longitudinal axis of thesegments of substantially 100% to substantially 131% of the nominal tubeoutside diameter, and the horizontally adjacent segments having agenerally vertical center-to-center spacing of substantially 110% tosubstantially 300% of the nominal tube outside diameter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe preferred embodiments of the invention, will be better understoodwhen read in conjunction with the appended drawings. For the purpose ofillustrating the invention, there are shown in the drawings embodimentswhich are presently preferred. It should be understood, however, thatthe invention is not limited to the precise arrangements andinstrumentalities shown.

FIG. 1 is an isometric view of one embodiment of a serpentine finnedtube according to the present invention used with other such finnedtubes in a coil assembly of an evaporative heat exchange apparatus.

FIG. 2 is an enlarged view of a portion of the serpentine tube of FIG.1, showing the area in FIG. 1 within the circle designated “FIG. 2.”

FIG. 3 is a vertical cross-section view taken along lines 3-3 of theembodiment of FIG. 2.

FIG. 4 is an end elevation view taken along the left-hand end of FIG. 1,showing a serpentine tube having a generally vertical plane extending90° into the plane of the drawing sheet.

FIG. 5A is a first embodiment view, partly in end elevation and partlyin vertical cross-section, of a portion of four tubes of a plurality ofserpentine tubes of a coil assembly, taken along lines 5-5 of theembodiment of FIG. 1, showing the generally elliptical segments havingtheir major axes generally vertically aligned and generally parallel tothe plane of the return bends when the tubes are generally verticallyoriented as shown with respect to the tube in FIG. 4.

FIG. 5B is a second embodiment view, partly in end elevation and partlyin vertical cross-section, of a portion of four tubes of a plurality ofserpentine tubes of a coil assembly, taken along lines 5-5 of theembodiment of FIG. 1, showing generally elliptical segments having theirmajor axes of adjacent tubes on different levels angled in oppositedirections with respect to each other and to the plane of the returnbends as shown in FIG. 4.

FIG. 6 is an isometric view of one embodiment of an exemplary coilassembly made using the finned tubes of the present invention.

FIG. 6A is a schematic side elevation drawing of the embodiment of theexemplary coil assembly of FIG. 6 made using serpentine finned tubes ofthe present invention.

FIG. 6B is a schematic side elevation drawing of an alternativeembodiment of an exemplary coil assembly made using the finned tubes ofthe present invention.

FIG. 6C is a schematic side elevation drawing of another alternativeembodiment of an exemplary coil assembly made using the finned tubes ofthe present invention.

FIG. 7 is a schematic, vertical cross-section view of a first embodimentof an induced draft, counterflow, evaporative heat exchanger includingan arrangement of two finned tube coil assemblies of the presentinvention within the evaporative heat exchanger.

FIG. 8 is a schematic, vertical cross-section view of an embodiment of aforced draft, counterflow, evaporative heat exchanger including anarrangement of two finned tube coil assemblies of the present inventionwithin the evaporative heat exchanger, with some typical componentsremoved for the sake of clarity.

FIG. 9 is a schematic, vertical cross-section view of an embodiment ofan induced draft evaporative heat exchanger including an arrangement ofa finned tube coil assembly of the present invention located directlybelow a direct contact heat transfer media section including wet deckfill within the evaporative heat exchanger, with some typical componentsremoved for the sake of clarity.

FIG. 10 is a schematic, vertical cross-section view of anotherembodiment of an induced draft evaporative heat exchanger including anarrangement of a finned tube coil assembly of the present inventionlocated directly above a direct contact heat transfer media sectionincluding wet deck fill within the evaporative heat exchanger, with sometypical components removed for the sake of clarity.

FIG. 11 is a schematic, vertical cross-section view of an embodiment ofan induced draft, counterflow evaporative heat exchanger including anarrangement of a finned tube coil assembly of the present inventionlocated in a spaced configuration below fill within the evaporative heatexchanger, with some typical components removed for the sake of clarity.

FIG. 12 is a graph of results of testing of various embodiments of anevaporative heat exchanger using coil assemblies of the presentinvention as compared to other types of coil assemblies under equivalentconditions using test procedures as explained hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described with reference to the drawings,where like numerals indicate like elements throughout the several views,and initially with reference to FIGS. 1-4, 5A and 5B showing embodimentsof a finned tube, together with FIGS. 6, 6A, 6B and 6C, showing variousembodiments of a coil assembly made using a number of the finned tubes,as well as FIG. 7, showing one embodiment of an exemplary evaporativeheat exchange apparatus containing the coil assembly of the finned tubesof the present invention.

While the preferred embodiments of the invention use finned tubes of thepresent invention for all of the tubes in a coil assembly of anevaporative heat exchange apparatus to provide the greatest advantagesand benefits of the invention, and are the embodiments described indetail hereinafter, other embodiments of the invention include using atleast one finned tube of the present invention in a coil assemblytogether with other, non-finned tubes in such a coil assembly.Preferably a plurality of finned tubes, such that at least some, morepreferably the majority, and most preferably as mentioned above, all ofthe tubes in a coil assembly for an evaporative heat exchange apparatusare the finned tubes of the present invention. When finned tubes areused in such a coil assembly together with non-finned tubes, the finnedtubes are used in any desired arrangement of finned and non-finnedtubes, but preferably and without limitation, the finned tubes mayusually be arranged to be on the top portion of a coil assembly and thenon-finned tubes may be on the bottom portion of the coil assembly.

The basic component of the present invention is a finned tube 10,preferably but not exclusively in the form of a serpentine tube bestseen in FIGS. 1-4, formed to provide the advantages of the inventionwhen combined with other such finned tubes into a coil assembly 24 (seeFIGS. 6 and 6A). The coil assembly 24 has a major plane 25, that in turnis used in an evaporative heat exchange apparatus, such as evaporativeheat exchanger 26, for example (see FIG. 7). When the finned tube 10 isin the preferred form of a serpentine tube, it has a plurality ofgenerally straight segments 12 that have a longitudinal axis 13 andwhich are interconnected by return bends 16. The tubes 10 may be made ofany heat-conductive metal, such as galvanized steel, stainless steel,copper, aluminum or the like. Stainless steel and galvanized steel,where the zinc is applied to the steel to form galvanized steel aftertubes are assembled into a coil assembly 24, are the presently preferredmaterials for the tubes 10 for most evaporative heat exchangeapplications.

The return bends 16 may be integrally and unitarily formed with thesegments 12 to form the tubes 10. Alternatively, the fins can beincluded on the segments 12 and the return bends 14, having connectorend portions 16 can be connected to connector end portions 18 of thesegment 12 after fins 20 are formed on the outer surface of the segments12. The connecting end portions 16 of the return bend 14 match the shapeand are typically slightly larger in cross-sectional area than theconnecting end portions 18 of the segments 12, such that the connectingend portions 18 of the segments fit within the connecting end portions16 of the return bend 14, and may be conveniently substantially sealedin a substantially liquid-tight and preferably substantially gas-tightmanner, such as by welding the connecting end portions 16 and 18together. Alternatively, the connecting end portions 16 of the returnbends 14 match the shape and may be slightly smaller in cross-sectionalarea than the connecting end portions 18 of the segments 12, such thatthe connecting end portions 18 of the segments fit over the connectingend portions 16 of the return bend 14, and may be convenientlysubstantially sealed in a substantially liquid-tight and preferablysubstantially gas-tight manner, such as by welding the connecting endportions 16 and 18 together. The connecting end portions 16 and 18 mayhave a generally elliptical or other cross-sectional shape. Preferably,for ease of manufacture and handling, the connecting end portions 16 and18 have a generally circular cross-sectional shape, such that it iseasier to orient and connect together the connecting end portions 16 and18, and so that uniform return bends 14 can be used that preferably havea generally circular cross-sectional shape throughout their curvedlength from one connecting end portion 16 to the opposite connecting endportion 16. However, if desired, such as for creating a more tightlypacked coil assembly of a plurality of generally horizontally arrangedtubes 10, the return bends may have a generally ellipticalcross-sectional shape, where major axes of the ellipses of the body ofthe return bends 14 between the connector end portions 16 are orientedin a generally vertical direction, for most applications within anevaporative heat exchanger. Alternatively, the return bends 14 may havea kidney-shaped cross-section throughout their length, with or withoutkidney-shaped connecting end portions 16 if the connecting end portions18 of the segments 12 have matching kidney-shaped cross-sections. It ispreferred to connect the return bends 14 to the segments 12 after thefins 20 have been applied to the segments, for ease of manufacture.

The tubes 10 are assembled into a coil assembly 24, best seen in FIGS. 6and 6A, where the tubes 10 are serpentine tubes. Typically, a coilassembly 24 has a generally rectangular overall shape retained in aframe 28, and is made of multiple serpentine tubes 10, where thesegments 12 are generally horizontal and closely spaced and arranged inlevels in planes generally parallel to the major plane 25 of the coilassembly 24. The coil assembly 24 has an inlet 30 connected to an inletmanifold or header 32, which fluidly connects to inlet ends of theserpentine tubes 10 of the coil assembly, and an outlet 34 connected toan outlet manifold or header 36, which fluidly connects to the outletends of the serpentine tubes 10 of the coil assembly. Although the inlet30 is shown at the top and the outlet 34 is shown at the bottom of thecoil assembly 24, the orientation of the inlet and outlet could bereversed, such that the inlet is at the bottom and the outlet is at thetop, if desired. The assembled coil assembly 24 may be moved andtransported as a unitary structure such that it may be dipped, ifdesired, if its components are made of steel, in a zinc bath togalvanize the entire coil assembly.

FIG. 6B is a schematic side elevation drawing of another alternativeembodiment of an exemplary coil assembly 24 made using the finned tubes10 of the present invention, where the finned tubes 10 are generallystraight tubes that extend across the major plane 25 (not shown). Inthis embodiment, an inlet 30 for the internal heat transfer or processfluid is connected to an inlet manifold or header 32. The internal fluidflows from the inlet manifold or header 32 into a plurality of finnedtubes 10 that are fluidly connected at one end to the inlet manifold orheader 32 at an upper level and into a second, upper manifold or header33A to which the opposite ends of the upper level finned tubes 10 arefluidly connected. The internal fluid then flows from the second, uppermanifold or header 33A through a lower level of finned tubes 10 fluidlyconnected at one end to the second, upper manifold or header 33A into athird, intermediate manifold or header 33B to which the opposite ends ofthe finned tubes 10 are fluidly connected. From the third, intermediatemanifold or header 33B, the internal fluid flows into a still lowerlevel of finned tubes 10 which are fluidly connected at one end to thethird, intermediate manifold or header 33B to a fourth, lower manifoldor header 33C to which the opposite ends of the finned tubes 10 arefluidly connected. Then the internal fluid flows from the fourth, lowermanifold or header 33C to which the one end of the lowest level of thefinned tubes 10 are fluidly connected to an outlet manifold or header 36to which the opposite ends of the finned tubes 10 are fluidly connected.An outlet 34 for the internal heat transfer or process fluid isconnected to the outlet manifold or header 36. As described aboveregarding the embodiment of FIGS. 6 and 6A, if desired for particularuses, the flow of the internal fluid can be reversed, such that thedescribed inlet 30 would be an outlet and the described outlet 34 wouldbe the inlet.

FIG. 6C is a schematic side elevation drawing of an alternativeembodiment of an exemplary coil assembly 24 made using the finned tubes10 of the present invention, where the finned tubes 10 are generallystraight tubes that extend across the major plane 25 (not shown) andfluidly connect directly at respective opposite ends to an inletmanifold or header 32 and to an outlet manifold or header 36. An inlet30 for the internal heat transfer or process fluid is connected to theinlet manifold or header 32. An outlet 34 for the internal heat transferor process fluid is connected to the outlet manifold or header 36. Asdescribed above regarding the embodiment of FIGS. 6, 6A and 6B, ifdesired for particular uses, the flow of the internal fluid can bereversed, such that the described inlet 30 would be an outlet and thedescribed outlet 34 would be the inlet.

The segments 12 of the finned tubes 10 shown in FIGS. 6 and 6A and thegenerally straight finned tubes 10 as shown in FIGS. 6B and 6C haveexternal fins 20, which are preferably spiral fins, that contact theouter surface of the segments 12. The fins may be serrated, may haveundulations or corrugations or may be of any other desired well-knownstructure. If desired, collars 22 may be integrally and unitarily formedwith the fins 20, where the collars 22 provide a direct and securecontact with the surface of the tubes 10 or segments 12 over a greatersurface area than if only the edges of the fins 20 were in contact withthe outer surface of the tubes 10 or segments 12. The fins 20 andcollars 22 may be formed simultaneously on the tubes 10 or segments 12using commercially available equipment in a manner known to thoseinvolved with producing filmed tubes, and especially spiral finnedtubes. Alternatively, the fins 20, with or without collars 20 may beapplied individually onto the outer surface of the tubes 10 or segments12, and then secured, such as by welding, into place, but this is anexpensive and labor intensive manner of applying the fins 20 to thetubes 10 or segments 12.

Preferably, the fins 20 are applied spirally in a continuous manner tothe tubes 10 or segments 12 by conventional equipment. The fins 20 areformed from a band of metal of the same type as used in for the tubes10, and the band is fed from a source of the band at a rate and in amanner to spirally wrapped around the tube 10 or segment 12 as the tube10 or segment 12 is advanced longitudinally along and rotated around itslongitudinal axis 13 through the spiral fin forming equipment. As thefins 20 are wrapped around the tube 10 or segment 12, the inner radiusof the fins 20 buckles while the outer radius does not, which createsminor corrugations or indentations in the fins themselves. This bucklingoccurs in a regular, repeating process in a left-to-right pattern toform undulations in and out of the plane of the material used to formthe fins, not shown in FIGS. 2 and 3.

If collars 22 are desired, the band of metal of the same type as used infor the tubes 10, is fed from a source of the band at a rate and in amanner to be bent longitudinally to provide a flat portion that becomesthe collars 22 and an upstanding portion that becomes the fins 20. Thebent metal band is spirally wrapped around the segments 12 as thesegments 12 are advanced longitudinally along and rotated around theirlongitudinal axis 13 through the spiral fin forming equipment. When thestrip of metal is spirally applied to the segments to form the fins 20with collars 22, the fins 20 typically have undulations in and out oftheir plane, rather than straight as shown in FIGS. 2 and 3 for the easeof illustration, while the collars 22 are flat against the surface ofthe segments 12, resulting from the metal deformation during theapplication of the strip of metal to the advancing and rotatingsegments.

FIGS. 5A and 5B show respective first and second embodiments, partly inend elevation and partly in vertical cross-section, of a portion of fourserpentine tubes 10A or 10B, for FIGS. 5A and 5B, respectively, of aplurality of tubes 10 of a coil assembly 24, taken along lines 5-5 ofthe embodiment of FIG. 1. As shown, starting from the left-hand side ofeach of FIGS. 5A and 5B, the second and fourth tubes are shown in apreferred orientation as being staggered in height, or vertically (asshown, lower), with respect to their next generally horizontallyadjacent first and third tubes. FIGS. 5A and 5B also illustratealternative embodiments of orientations of the major axes of thegenerally elliptical segments 12A of serpentine tubes 10A in FIG. 5A andthe generally elliptical segments 12B of serpentine tubes 10B in FIG.5B. Otherwise, the embodiments of FIGS. 5A and 5B are similar to eachother. In FIGS. 5A and 5B, the cross-section of FIG. 1 was selected suchthat the fins are not shown or described for the sake of clarity, butthe orientations of the major and minor axes of the generally ellipticalsegments should be understood as relating to the entire length of thefinned segments 12 until they connect with or are unitarily formed withthe return bends 14A and 14B. Although each of the return bends 14A and14B is shown as having a circular cross-sectional shape, as explainedabove, the return bends 14A and 14B may alternatively have a generallyelliptical cross-sectional shape, a generally kidney-shapedcross-sectional shape, or other cross-sectional shape. For ease ofexplanation, the orientation of the major axes of the generallyelliptical finned segments 12A and 12B will be described in thepreferred embodiment of the serpentine tubes 10 as shown in theembodiment illustrated in FIGS. 6 and 6A, but in principle, the sameorientation can be and, preferably, is provided for the generallystraight and generally elliptical finned tubes 10 used in a coilassembly such as the coil assemblies shown in FIGS. 6B and 6C.

In both FIGS. 5A and 5B, the segments 12A or 12B of adjacent tubes aregenerally vertically spaced from each other within planes generallyparallel to the major plane 25 of the coil assembly 24 at respectiveupper generally horizontal levels L1A and L1B and respective lowergenerally horizontal levels L2A and L2B. Thus, the segments 12A or 12Bof adjacent tubes 10A or 10B are in planes generally parallel to themajor plane 25 and are staggered and spaced with respect to each othergenerally vertically to form a plurality of staggered generallyhorizontal levels in which every other segment is aligned in the samegenerally horizontal level generally parallel to the major plane 25.

In the first embodiment of FIG. 5A, the generally elliptical segments12A have their major axes generally vertically aligned and generallyparallel to the plane of the return bends 14A when the tubes 10A aregenerally vertically oriented as shown with respect to the tube 10 inFIG. 4. This alignment or orientation is regardless of whether thesegments are on an upper generally horizontal vertical level L1A or alower horizontal level, such as the next adjacent generally horizontallevel L2A.

In the second embodiment of FIG. 5B, the generally elliptical segments12B have their major axes of the tubes 10B on the different, nextadjacent generally horizontal levels L1B and L2B, angled in oppositedirections with respect to the plane of the return bends 14B when thetubes 10B are generally vertically oriented as shown with respect to thetube 10 in FIG. 4. As shown in FIG. 5B, in a preferred embodiment wherethe major axes of the segments 12 are oriented in opposite directions onadjacent horizontal levels, the angle of all of the major axes on afirst generally horizontal level L1B is about 20° from the plane of thereturn bends and the angle of all of the major axes on the next adjacentgenerally horizontal level L2B is about 340° from the plane of thereturn bends. In this configuration, each horizontal level L1B, themajor axes of all of the segments 12B are oriented in the same angleddirection and on the next adjacent lower level L2B, the major axes ofall the segments are oriented in the same angled direction, but in anopposite angled orientation from the angled orientation of the majoraxes in level L1B. Where the major axes are angled in oppositedirections on adjacent horizontal levels, they are sometimes known as a“ric-rac” arrangement or orientation, and this term is used in the Tablebelow to designate this type of arrangement or orientation. If desired,however, on each level L1B or L2B, the major axes of the segments withinthe same generally horizontal level may be angled in oppositedirections.

Thus, as represented in FIGS. 5A and 5B, the major axes of the finnedsegments 12A or 12B on a first generally horizontal level L1A or L1B,respectively, may be 0° to about 25° degrees from the plane of thereturn bends and the angle of the major axes of the finned segments 12Bor 12A, respectively, on the next adjacent generally horizontal levelL2B or L2A, respectively, may be about 335° to 360° from the plane ofthe return bends. FIG. 4 shows the oppositely angled major axes of thefinned segments 12 as described with respect to FIG. 5B for a completeserpentine tube 10.

The return bends 14, 14A and 14B are shown as being generally circularin cross-section. The outside diameter of the circular cross-section ofthe return bends substantially equals the nominal tube outside diameterthat is an average of the lengths of the major and minor axes of thesegments 12, 12A and 12B having a generally elliptical cross-section.Preferably, but without limitation, the outside diameter of the returnbends and the nominal tube outside diameter are about and preferablysubstantially 1.05 inches (2.67 cm), where the wall thickness of thetubes forming the segments 12 and the return bends 14 is about 0.055inch (0.14 cm). The minor axis of the generally elliptical tube 10 orsegments 12, 12A and 12B is about 0.5 to about 0.9 times, and preferablyabout 0.8 times the nominal tube outside diameter. Thus, the generallyelliptical straight tubes 10 and segments 12, 12A and 12B having anominal tube outside diameter of 1.05 inches (2.67 cm), would have aminor axis length of about and preferably substantially 0.525 inch(1.334 cm) to about and preferably substantially 0.945 inch (2.4 cm),and preferably about and preferably substantially 0.84 inch (2.134 cm).Tubes 10 with these dimensions have been found to have a good balanceamong an appropriate inner diameter or dimensions to allow theprocessing fluid in the form of any desired gas or liquid to easily flowwithin the tubes 10, proximity of such processing fluid to the tube wallfor good heat transfer through the walls of the tubes with theelliptical cross-sectional shape that has a large effective surfacearea, and ability to provide an appropriate number of tubes 10 to bepacked into a coil assembly 24. The tubes are strong, durable and whenin serpentine form, able to be readily worked, including connecting thesegments 12 and return bends 14 and placement within a coil assembly 24.Depending on the environment and intended use of the evaporative heatexchangers, such as the evaporative heat exchanger 26, in which thefinned tubes 10 of the present invention are placed, the dimensions andcross-sectional shape of the tubes 10 may be varied considerably.

The spacing and orientation of the tubes 10 having the generallyelliptical cross-sectional shape or segments having the generallyelliptical cross-sectional shape within a coil assembly 24 are importantfactors for the performance of the evaporative heat exchanger containingthe coil assembly 24. If the spacing between segments 12 is too tight,air and water flow through and turbulent mixing within the coil assemblywill be adversely affected and fans with greater horsepower will beneeded and there will be an increased pressure drop. If the spacingbetween segments 12 is too great, then there will be less tubes persurface area of the major plane 25 of the coil assembly 24, reducing theheat transfer capacity, and there may be inadequate, as in insufficientfor example, mixing of the air and water, adversely affecting the degreeof evaporation, and thereby heat exchange. The orientation of thesegments 12, particularly with respect to the angle of the major axes ofthe segments, also affects the heat exchange ability of an evaporativeheat exchanger with which they are used.

The spacing of the fins 20 around the outer surface of the segments 12is critical. If the fin spacing is too close (too many fins per inch,for example), the ability of the external heat exchange liquid and theair to effectively mix turbulently is adversely affected and the fins 20may block the space externally of the coil assembly 24, such thatgreater air mover power is needed. Similar concerns involve the criticaldetermination of the height of the fins (the distance from the proximalpoint where the base of the fins 20 contact the outer surface of thesegments 12 and the distal tip of the fins). While higher fins havegreater surface area which the evaporating water may coat, longer finsmay block the air passage. Thicker fins 20 also have similar criticalconcerns. Thicker fins are more durable and are better able to withstandthe forces of water and air, as well as other material that may beentrained in either as they pass through a coil assembly, but thickerfins may also block the flow of water or air through the coil assemblyand would be more expensive to manufacture. All of these factorsadversely affect performance.

If the fin spacing is too great (not enough fins per inch, for example),the advantages of a sufficient number of fins 20 for the evaporativewater to coat would not be present and there may be an adverse effect onthe desired mixing of the water and air responsible for efficientevaporation. Similar concerns are present when the fin height is toolow, as there is not enough structure of the fins to be coated with thewater, and there may be less mixing of the water and air. Thinner finsmay not be sufficiently durable to withstand the hostile environment towhich they are subject in evaporative heat exchangers and if the finsare too thin, they could be bent during operation as they are subject tothe forces of both the water and air impacting them, adversely affectingflow of both the water and air. In addition, and more significantly,thinner fins transfer less heat.

The present invention was conceived and developed in view of theforegoing factors of tube shape, orientation, arrangement and spacing,and fin spacing, height and thickness, all of which must be carefullybalanced, and which was a difficult task requiring considerable testingand experimentation. Based on such work, the appropriate parameters oftube shape, arrangement, orientation and spacing, as well as finspacing, height and thickness were determined.

The orientation and spacing, within a coil assembly 24 and anevaporative heat exchanger, of the tubes 10 with their segments 12 andreturn bends 14 will be described primarily with reference to FIGS. 5Aand 5B. The center-to-center spacing D_(H) generally horizontally (whichwill be generally parallel to the major plane 25 in FIG. 6) andgenerally normal to the longitudinal axis 13 of the segments 12, 12A and12B is substantially 100% to substantially 131%, preferablysubstantially 106% to substantially 118%, and more preferablysubstantially 112% of the nominal tube outside diameter. The verticalstraight tube or segment spacing D_(V) generally is not as critical tothe performance of an evaporative heat exchanger as the horizontal tubeor segment spacing D_(H). The segments 12, 12A and 12B have a generallyvertical center-to-center spacing of substantially 110% to substantially300% of the nominal tube outside diameter, preferably substantially 150%to substantially 205% of the nominal tube outside diameter, and morepreferably, substantially 179% of the nominal tube outside diameter.This generally vertical center-to center spacing is indicated by thedistance D_(V) between the upper generally horizontal levels L1A and L1Band the lower generally horizontal levels L2A and L2B, respectively.

These parameters may be applied as follows to the presently preferredembodiment, where the nominal tube outside diameter is substantially1.05 inches (2.67 cm). The center-to-center spacing D_(H) of the finnedstraight tubes 10 or segments 12, 12A and 12B of the serpentine finnedtubes 10 would be substantially 1.05 inches (2.67 cm) to substantially1.38 inches (3.51 cm), preferably substantially 1.11 inches (2.82 cm) tosubstantially 1.24 inches (3.15 cm), and more preferably substantially1.175 inches (2.985 cm). The finned tubes 10 or the finned segments 12,12A and 12B would have a generally vertical center-to-center spacingD_(V) of substantially 1.15 inches (2.92 cm) to substantially 3.15inches (8.00 cm), preferably substantially 1.57 inches (3.99 cm) tosubstantially 2.15 inches (5.46 cm), and more preferably substantially1.88 inches (4.78 cm). In some embodiments, the major axes of the finnedtubes 10 or the finned segments 12, 12A are oriented substantiallyvertically, so that they are generally parallel to the plane of thereturn bends 14 as shown in FIG. 4. In other embodiments, the major axesof the finned tubes 10 or the finned segments 12B may be greater than 0°to about 25°, and preferably about 20°, from the plane of the returnbends 14 and the angle of the major axes of the finned tubes 10 or thefinned segments 12B on the next vertically adjacent generally horizontallevel, may be about 335° to less than 360°, and preferably about 340°from the plane of the return bends 14, such that the major axes of thefinned tubes 10 or the finned segments 12 are oriented in oppositedirections on vertically adjacent horizontal levels.

The parameters relating to the fins 20, namely fin spacing along thelongitudinal axis 13 of the segments 12, the fin height from the outersurface of the segments 12 and the fin thickness are as followsaccording to the present invention.

The fins 20 are preferably spiral fins and have a spacing ofsubstantially 1.5 to substantially 3.5 fins per inch (2.54 cm) along thelongitudinal axis 13 of the segments 12, preferably substantially 2.75to substantially 3.25 fins per inch (2.54 cm) and more preferablysubstantially 3 fins per inch (2.54 cm). Expressed alternatively, thecenter-to-center distance between the fins is therefore, respectively,substantially 0.667 inch (1.694 cm) to substantially 0.286 inch (0.726cm), preferably substantially 0.364 inch (0.925 cm) to substantially0.308 inch (0.782 cm), and more preferably substantially 0.333 inch(0.846 cm).

The fins 20 have a height of substantially 23.8% to substantially 36% ofthe nominal tube outside diameter, preferably substantially 28% tosubstantially 33% of the nominal tube outside diameter, and morepreferably substantially 29.76% of the nominal tube outside diameter.These parameters may be applied as follows to the presently preferredembodiment, where the nominal tube outside diameter is substantially1.05 inches (2.667 cm). In this embodiment, the fins 20 have a height ofsubstantially 0.25 inch (0.635 cm) to substantially 0.375 inch (0.953cm), preferably substantially 0.294 inch (0.747 cm) to substantially0.347 inch (0.881 cm), and more preferably 0.3125 inch (0.794 cm).

The fins 20 have a thickness of substantially 0.007 inch (0.018 cm) tosubstantially 0.020 inch (0.051 cm), preferably substantially 0.009 inch(0.023 cm) to substantially 0.015 inch (0.038 cm), and more preferablysubstantially 0.01 inch (0.025 cm) to substantially 0.013 inch (0.033cm). As noted above in the “Definitions” section, dimensions for thethickness of the fins are for the fins on the finned tubes prior to anylater treatment of the finned tubes themselves or of any coil assemblycontaining them. Where the finned tubes or coil assembly are subjectedto a later treatment, typically by galvanizing steel finned tubes ormore typically, galvanizing the entire coil assembly containing them,the thickness of the fins increases by the thickness of the zinc coatingapplied during galvanization. Also typically, the fins aftergalvanization are thicker at a base proximal to the outer surface of thetube than at a tip of the fins distal from the outer surface of thetube. Because the fins are thicker after galvanizing, the spacingbetween the fins is reduced accordingly. Usually this is not of concernconcerning the thermal performance or heat capacity of the evaporativeheat exchangers and the rust or other corrosion inhibition of thegalvanizing is important in providing the finned tubes and coilassemblies with greater longevity than if they were not galvanized.

The coil assembly 24 of any desired configuration, such as shown in anyof FIG. 6, 6A, 6B or 6C, is then installed into an evaporative heatexchanger apparatus, such as evaporative heat exchanger 26, as shown inFIG. 7. Evaporative heat exchangers have many varied configurations, andseveral are shown schematically in FIGS. 7-11. Typical evaporative heatexchangers in which the coil assembly 24 of the present invention may beused are, for example without limitation, any of several available fromEvapco, Inc., such as Models ATWB or ATC, which may include thecomponents and operate as disclosed in Evapco, Inc.'s U.S. Pat.4,755,331. Evaporative heat exchange apparatus, though they manyvariations, have the basic structure and operation described below,initially with reference to FIG. 7.

FIG. 7 is a schematic, vertical cross-section view of an embodiment ofan induced draft, counterflow, evaporative heat exchanger 26, wherewater flows generally vertically downwardly and air flows generallyvertically upwardly through the plenum and coil assembly, including anarrangement of two finned tube coil assemblies 24 of the presentinvention within the evaporative heat exchanger. The evaporative heatexchanger 26 has a housing 38 enclosing a plenum 40 having a generallyvertical longitudinal axis 42. One or more coil assemblies 24 aremounted within the plenum 40 such that the major plane 25 of each coilassembly is generally normal to the longitudinal axis 42 of the plenum.In this way, the generally vertical plane of the return bends 14 in thepreferred embodiment using serpentine tubes 10, as shown in FIG. 4 andas indicated by the generally vertical alignment of the tubes 10 in thecoil assemblies as shown in FIG. 7, are also generally normal to themajor plane 25 of the coil assemblies 24 and parallel to thelongitudinal axis 42 of the plenum. Based on this alignment, the finnedsegments 12, with their longitudinal axes 13, of the tubes 10 are alsoin generally horizontal staggered planes parallel to the major plane 25of the coil assemblies 24 and generally normal to the longitudinal axis42 of the plenum 40. If generally straight finned tubes 10 are used asshown in FIGS. 6B and 6C, then the finned tubes with their longitudinalaxes also are in generally horizontal staggered planes parallel to themajor plane 25 of the coil assemblies 24 and generally normal to thelongitudinal axis 42 of the plenum 40.

Air flows from the ambient atmosphere around the heat exchanger 26 viaair inlets 44 which may, and preferably do, have louvers, or morepreferably, selectively openable and closeable air inlet dampers 45 thatmay be closed or partially or fully opened based on various atmosphericand operating conditions, in a well-known manner, and to protect theplenum 40 from inclusion of unwanted objects. In the embodiment of FIG.7, air is drawn into the plenum 40, passes though the coil assemblies 24and exits an air outlet 46 by the action of an air mover located in anair outlet housing 50. The air mover in this embodiment is shown as afan 48, in the form of a propeller fan, which is preferred for use as aninduced draft fan to draw air from the ambient atmosphere. Other typesof fans, such as centrifugal fans, could be, but usually are not used asinduced draft fans. A grating or screen (not shown) is placed over thefan 48 for safety and to keep debris away from the fan 48 and out of theevaporative heat exchanger 26.

A bottom wall of the evaporative heat exchanger 26, together with theadjoining front, back and side walls, defines a sump 52 for the water orother external heat exchange liquid. If desired, a drain pipe with anappropriate valve and a fill pipe with an appropriate valve (none ofwhich is shown) may be included for draining and filling or replenishingthe sump 52. Water in the sump 52 is circulated to a liquid distributorassembly 54, which when turned on distributes, via spray nozzles,orifices in a pipe or via other known devices and techniques, the wateras the evaporative heat transfer liquid above the coil assemblies 24.The distributor assembly 54 is connected to one end of a conduit 56 influid connection at the other end to the water in the sump. Thedistributor assembly 54 is activated or turned on typically when a pump58 is turned on to pump water from the sump 52 to the distributorassembly 54 through the conduit 56.

The evaporative heat exchanger 26 also preferably includes drifteliminators 60 above the liquid distributor assembly 54 and below thefan 48 and air outlet 46. The drift eliminators very significantlyreduce water droplets or mist entrained in the air exiting the outlet46. Many drift eliminators of various materials are availablecommercially. The presently preferred drift eliminators are PVC drifteliminators available from Evapco, Inc. as disclosed in Evapco, Inc.'sU.S. Pat. No. 6,315,804, the disclosure of which is hereby incorporatedby reference herein in its entirety.

In operation, as air is drawn into the plenum 40 through the air inlets44 and any associated louvers or dampers 45, it is also drawn throughthe coil assemblies 24. Water is distributed over the coil assemblies 24by the liquid distributor 54. As the air travels upwardly through thecoil assemblies 24 it is mixed with the water, with an appropriatedegree of turbulence as provided by the orientation and arrangement ofthe finned segments 12 having the fins 20 with the characteristics,dimensions and parameters disclosed above. The water coats the outersurfaces of the tubes 10, including the segments 12 having the generallyelliptical cross-sectional shape, as well as the fins 20. The air causesthe water to evaporate, thereby cooling the water, such that the cooledwater exchanges heat with the tubes 10 of the coil assembly and theprocess fluid contained internally within the tubes 10. Water ultimatelypasses through the coil assemblies 24 and is collected in the sump 52,and recycled into the liquid distributor 54 through the conduit 56 bythe pump. The air with any entrained water is drawn upwardly through thedrift eliminators 60, whereby most, and preferably almost all, of thewater is removed from the air stream, before the air is exhaustedthrough the air outlet 46 by the fan 48.

As noted above, the coil assemblies 24 having the finned tubes 10 of thepresent invention may be used in a large variety and types ofevaporative heat exchange apparatus. FIGS. 8-11 schematically illustratea small sample of such various evaporative heat exchangers, with sometypical components shown in FIG. 7 removed for the sake of clarity. InFIGS. 8-11, components that are shown and that are the same as those inFIG. 7 are not described again, but are identified by like numerals usedin FIG. 7, except that a letter designation common to the embodiments ofeach of FIGS. 8-11 is used, where, for example, the coil assemblies 24Aare used in the evaporative heat exchanger 26A of FIG. 8, the coilassembly 24B is used in the evaporative heat exchanger 26B of FIG. 9,the coil assembly 24C is used in the evaporative heat exchanger 26C ofFIG. 10 and the coil assembly 24D is used in the evaporative heatexchanger 26D of FIG. 11. Any new components not used in a previous Fig.are identified by a different numeral.

FIG. 8 is a schematic, vertical cross-section view of an embodiment of aforced draft, counterflow, evaporative heat exchanger 26A including anarrangement of two finned tube coil assemblies 24A of the presentinvention within the plenum 40A of the evaporative heat exchanger. Here,compared to the induced draft evaporative heat exchanger 26 of FIG. 7,instead of using a propeller fan 48 mounted in an air outlet housing 50,the forced draft evaporative heat exchanger 26A of FIG. 8 uses acentrifugal fan 62 type of air mover to force air, entering the plenum40A within the housing 38A through a screen 47 covering the air inlet.The air is then forced generally vertically upwardly and through thecoil assemblies 24A, through which water is flowing generally verticallydownwardly. Thereafter, the air moves through the drift eliminators 60Aand out of the evaporative heat exchanger 26A through the air outlet46A. The centrifugal fan 62 is typically mounted within a lower portionat one side of the housing 38A adjacent an air inlet typically coveredby a screen 47. The sump for the water is not shown in FIG. 8, but wouldbe present below the coil assemblies 24A such that the water in the sumpis blocked from reaching the centrifugal fan 62.

FIG. 9 is a schematic, vertical cross-section view of an embodiment ofan induced draft evaporative heat exchanger 26B including an arrangementof a finned tube coil assembly 24B of the present invention locateddirectly below a direct contact heat transfer media section includingwet deck fill 64, described below, within the plenum 40B of theevaporative heat exchanger. In the evaporative heat exchanger 26B ofFIG. 9, air is drawn into the plenum 40B through an air inlet 44B andany associated louvers or dampers 45B, where the air inlet 44B islaterally adjacent to the coil assembly 24B. The evaporative heatexchanger 26B of FIG. 9 differs in a first respect from the evaporativeheat exchanger 26 of FIG. 7, in that the air is drawn through the coilassembly 24B in a direction generally normal, transverse or horizontallywith respect to the generally vertical downwardly flow of waterexternally through the coil assembly 24B, known in the industry as acrossflow arrangement. The mixing and turbulence of the air and waterexternally through the coil assembly 24B in a crossflow arrangement issomewhat different than but still quite effective, compared to themixing and turbulence of the air and water externally through the coilassembly 24 of FIG. 7 in a counterflow arrangement.

The evaporative heat exchanger 26B of FIG. 9 differs in a second respectfrom the evaporative heat exchanger 26 of FIG. 7 in that the evaporativeheat exchanger 26B of FIG. 9 includes a direct contact heat exchangesection containing wet deck fill 64 below the liquid distributor 54B andabove the coil assembly 24B, which provides direct, evaporative heatexchange when the air flow and the evaporative water or other coolingliquid come into direct contact with each other and are mixed with somedesired degree of turbulence within the wet deck fill 64 resulting inadditional evaporative cooling. The turbulent mixing of the air andwater in the wet deck fill 64 allows for greater heat transfer betweenthe air and water, but the benefits of the increased turbulent mixing inthe wet deck fill 64 should not be overcome by potential adverse effectson the energy requirements of a larger fan motor or fan size or air flowreduction. As noted above, there is a fine balance among these factorswhen deciding whether and what type of wet deck fill heat transfer mediato use. That is why the use of the wet deck fill 64 is optional inevaporative heat exchangers using the coil assembly of the presentinvention. The wet deck fill may be any standard fill media, such asplastic fill, typically PVC, as well as wood or ceramic fill media, orany other fill media known in the art. The presently preferred fillmedia is Evapco, Inc.'s EVAPAK® PVC fill, disclosed in Evapco, Inc.'sU.S. Pat. No. 5,124,087, the disclosure of which is hereby incorporatedby reference herein, in its entirety. When wet deck fill 64 is used, itmay be located above the coil assembly 24B as shown in FIG. 9, or belowthe coil assembly 24C as shown in FIG. 10, since in either location, theadditional heat transfer in the wet deck fill 64 will furtherevaporatively cool the water draining into the sump 52B or 52C.

In the embodiment of FIG. 9, louvers 65 are built into the inlet side ofthe wet deck fill 64, such that the air may be drawn through the louvers65 into the wet deck fill in a crossflow manner as described above withrespect to the crossflow arrangement concerning the coil assembly 24B.

The embodiment of the evaporative heat exchanger 26B of FIG. 9 operatesas follows. Ambient air in the environment of the evaporative heatexchanger is drawn into the plenum 40B through the air inlets 44B andany associated louvers or dampers 45B, and in a crossflow mannerexternally through the coil assembly 24B, though which water, pre-cooledin the wet deck fill 64 of the direct contact heat exchange section,externally flows generally vertically downwardly. Ambient air is alsodrawn into the wet deck fill 64 in a crossflow manner with respect tothe water flowing generally vertically downwardly through the louvers65, where the water is evaporatively cooled before it contacts the coilassembly 24B below the wet deck fill 64. The air is then drawn from thewet deck fill 64 into the plenum 40B.

Water is distributed over the wet deck fill 64 by the liquid distributor54B where it is initially cooled evaporatively by mixing with the airflowing through the wet deck fill 64 before draining into the coilassembly 24B where it is turbulently mixed with the air and thereafteris drained from the coil assembly 24B and collected in the sump 52B. Thewater is recycled from the sump 52B into the liquid distributor 54Bthrough the conduit 56B by the pump 58B. The air, with any entrainedwater, in the plenum 40B is drawn upwardly through drift eliminators 60(not shown in FIG. 9) by the fan 48B in the air outlet housing 50B,before the air is exhausted through the air outlet 46B.

FIG. 10 is a schematic, vertical cross-section view of anotherembodiment of an induced draft evaporative heat exchanger 26C includingan arrangement of a finned tube coil assembly 24C of the presentinvention located directly above a direct contact heat transfer mediasection including wet deck fill 64C within the plenum 40C of theevaporative heat exchanger. The embodiment of the evaporative heatexchanger 26C of FIG. 10 operates as follows. One portion of ambient airin the environment of the evaporative heat exchanger is drawn into theapparatus through an inlet 44C at the top of the apparatus aligned abovethe coil assembly 24C and flows downwardly externally through the coilassembly in a generally vertical direction concurrent with the flow ofwater distributed over the coil assembly by the liquid distributor 54C.Another portion of ambient air is also drawn into apparatus through thedirect contact heat exchange section containing the wet deck fill 64Cthrough the optional louvers 65C. The air traveling through the wet deckfill 64C moves in a crossflow manner to water draining generallyvertically from the coil assembly 24C.

Water is distributed over the coil assembly 24C by the liquiddistributor 54C where it is mixed with the concurrently flowing air,thereby being cooled evaporatively in the coil assembly, exchanging heatwith the coil assembly 24C, before draining into and through the wetdeck fill 64C. In the wet deck fill 64C, the water is furtherturbulently mixed with the cross-flowing air where it is furtherevaporatively cooled, and thereafter is drained from the wet deck fill64C and collected in the sump 52C. The water is recycled from the sump52C into the liquid distributor 54C through the conduit 56C by the pump58C. The air with any entrained water is drawn into the plenum 40C andthen upwardly through drift eliminators 60 (not shown in FIG. 10) by thefan 48C in the air outlet housing 50C, before the air is exhaustedthrough the air outlet 46C.

FIG. 11 is a schematic, vertical cross-section view of an embodiment ofan induced draft, counterflow, evaporative heat exchanger 26D includingan arrangement of a finned tube coil assembly 24D located in a spacedconfiguration below wet deck fill 64D within the plenum 40D in thehousing 38D in the evaporative heat exchanger.

The embodiment of the evaporative heat exchanger 26D of FIG. 11 operatesas follows. Air in the environment of the evaporative heat exchanger isdrawn into the plenum 40D through the air inlets 44D and any associatedlouvers or dampers 45D, and then is drawn into the wet deck fill 64D ina counterflow manner with respect to the water flowing generallyvertically downward through the wet deck fill 64D. The liquiddistributor 54 (not shown in FIG. 11), located above the wet deck fill64D and below the drift eliminators (not shown in FIG. 11), distributesthe water over the wet deck fill 64D where it is turbulently mixed withthe air, thereby being cooled evaporatively. Then, the cooled waterdrains over the coil assembly 24D, exchanging heat with the coilassembly 24D, before draining into and being collected in the sump 52D.If desired, the water draining from the wet deck fill 64D may beconcentrated to flow directly over the coil assembly 24D as disclosed inEvapco, Inc.'s U.S. Pat. No. 6,598,862, the disclosure of which ishereby incorporated by reference herein, in its entirety, to moreefficiently direct the cooled water to the coil assembly 24D. The wateris recycled from the sump 52D into the liquid distributor 54 through theconduit 56 (not shown in FIG. 11) by the pump 58 (not shown in FIG. 11).The air with any entrained water is drawn upwardly through drifteliminators by the fan 48D in the air outlet housing 50D, before the airis exhausted through the air outlet 46D.

The performance of evaporative heat exchange apparatus is measured bythe amount of heat transfer, typically but not exclusively duringcooling. The measurements are affected by several factors. First, themeasurements are affected by the amount and temperature of the processfluid flowing internally though the tubes 10 of the apparatus coilassembl(ies) 24 and the water or other cooling liquid flowing externallythrough the coil assembly. The flow rates are measured using flow metersand the temperature is measured using thermometers. The rate andtemperature of the air flowing through the system is also significant,as well as the force required to drive the air mover 48 that moves theair through the apparatus. The air flow is typically measured by ananemometer in feet per minute through a tube, although other well-knownair flow measuring devices could also be used, and is typicallydetermined by the rating of the motor driving the fan of the air mover,usually expressed in horsepower (HP).

In one embodiment of the evaporative heat exchange apparatus using thecoil assemblies 24 having the finned tubes 10 of the present invention,typically, but without limitation, the process fluid, in the form ofwater, is pumped into the inlet 30 and flows internally through the coilassembly at a rate of approximately 0.75 gpm to approximately 16.5 gpmper tube present in the coil assemblies, and preferably approximately 10gpm per tube. The amount and rate of water that passes externallythrough the coil assembl(ies) 24 supplied through the water supplyconduit 56 as distributed by the liquid distributor 54 is approximately1.5 gpm/sq. ft. to approximately 7 gpm/sq. ft. of coil plan areadetermined with respect to the major plane 25, and is preferablyapproximately 3 gpm/sq. ft. to approximately 6 gpm/sq. ft. Evaporativeheat exchange apparatus using the coil assemblies 24 having the finnedtubes 10 of the present invention typically, but without limitation,have an air flow rate of approximately 300 feet per minute toapproximately 750 feet per minute, and preferably approximately 600 feetper minute to approximately 650 feet per minute. The power of the fanmotors is dependent upon the size of the evaporative heat exchangerhousing, the size of the coil assemblies used, the number andconfiguration of tubes in the coil assemblies, the number of coilassemblies used, the presence and orientation of any optional wet deckfill, the size and type of fan used, and several other factors, so noabsolute values can be presented for the power of the fan motorsrequired. In general, and without limitation, the power of the fanmotors varies within a very broad range, such as approximately 0.06 HPto approximately 0.5 HP per square foot of plan area of the coilassemblies used in the evaporative heat exchangers, corresponding to thearea of the major plane 25 coextensive with the length and width of thecoil assembly.

In evaporative heat exchange apparatus using the finned tube coilassemblies 24 of the present invention, performance has been shown to beenhanced by an increased air flow rate even compared to similar coilassemblies using tubes having segments 12 with a generally ellipticalcross-sectional shape but not containing fins 20 as in the presentinvention. In view of the space occupied by the fins 20 on the segments12 of the tubes 10 used in coil assemblies 24 of the present invention,it would have been expected that the air flow rate would have decreased,as the fins 20 would have been expected to block the flow of both airand water, so that it was unexpected and surprising when the air flowrate increased. The increase in air flow rate provided a surprisingenhancement of the thermal performance in evaporative heat exchangeapparatus using the coil assemblies with the finned tubes 10 of thepresent invention.

The enhanced thermal performance of evaporative heat exchange apparatususing the coil assemblies 24 having finned tubes of the presentinvention will be described in greater detail with respect to thefollowing non-limiting test procedure whereby various coil assemblieswere tested, including those of the present invention, under equivalenttest conditions.

The test procedure included mounting various single coil assemblies inan Evapco, Inc. Model ATWB induced draft, counterflow, evaporativecooler in a test facility. The general arrangement of the Model ATWBinduced draft, counterflow, evaporative cooler is shown in FIG. 7,except that only one coil assembly 24 was used, instead of two coilassemblies 24 as shown in FIG. 7. The tested coil assemblies all had aplan area of 6 feet (1.83 m) long (corresponding to serpentine tubeshaving segments with return bends fitting within frames of this lengthwith the appropriate spacing) by 4 feet (1.22 m) wide (corresponding to37 adjacent tubes that were packed within frames of this width with theappropriate spacing) and had ten generally horizontal rows of segments12 with generally elliptical cross-sectional shapes connected by returnbends having a circular cross-sectional shape, where the major axes ofsegments were arranged in various orientations. All tested coilassemblies used tubes with return bends having an outside diameter ofsubstantially 1.05 inches (2.67 cm) and segments having a nominal tubeoutside diameter of substantially 1.05 inches (2.67 cm), with asubstantially horizontal center-to-center spacing D_(H) of 1.0625 inches(2.699 cm) (designated “Narrow” in the Table below) or 1.156 inches(2.936 cm) (designated “Wide” in the Table below) and a substantiallyvertical center-to-center spacing D_(V) of about 1.875 inches (4.763cm). One tested coil assembly had no fins 20 on the segments (Test ID“A” in the Table below and the graph of FIG. 12) and represented a baseline against which other finned coil assemblies were compared. Othertested coil assemblies identified in the Table below and the graph ofFIG. 12 had spiral fins 20 with the parameters of fin spacing and heightas described and claimed herein, and some had spiral fins 20 but nothaving the parameters of fin spacing and height as described and claimedherein. All of the coil assemblies including fins used fins of the samethickness, namely, 0.013 inch (0.033 cm), which is within the range offin thickness described and claimed herein. Certain other coilassemblies, namely, those having the parameters associated with the TestID “B” and “C” (tested in a different rig) and Test ID “D” (tested using5 HP motor) in the Table below and the graph of FIG. 12, were tested ina different manner, but the performance data presented in the graph ofFIG. 12 were derived using industry calculations for standardizingperformance data from apparatus of different configurations. Theperformance of the coil assemblies was tested over varying water flowrates internally through the coils of 60 gpm to 360 gpm, water flowrates externally through the coils of approximately 5.9 gpm per squarefoot, and air flow rates of 300 feet per minute (91.44 meters perminute) to 750 feet per minute (228.6 meters per minute), generated by afan driven by a 3 HP motor (except as noted above regarding Test ID“C”). The coil assemblies tested had the parameters as set forth in thefollowing Table:

Major Axes D_(H) Tube Fin Spacing Fin Height Test ID Orientation SpacingFins (Fins/Inch) (Inch) A 20° & 340° Wide No — — Ric-rac B 0° Wide Yes 30.25 C 20° & 340° Wide Yes 1.5 0.3125 Ric-rac D 0° Narrow Yes 3 0.3125 E20° & 340° Wide Yes 3 0.3125 Ric-rac F 0° Wide Yes 3 0.3125 G 20° & 340°Wide Yes 1.5 0.5 Ric-rac H 20° & 340° Wide Yes 3 0.5 Ric-rac

FIG. 12 is a graph of results of testing of the coil assembliesidentified in the Table in the evaporative heat exchanger under the sameconditions set forth in the procedure described above, with respect topreferred internal process fluid (water) flow rates from 6 to 9.8 gpmper tube (where each tube is identified as a “circuit” in the x-axislegend on the graph. The graph show curves based on the heat transferredas measured in thousands of BTU/hour (MBH) versus the water flowinternally through the coil assembly in gallons/minute/tube (GPM). Eachcurve A to H in FIG. 12 corresponds to the respective coil assembly A toH of the above Table.

With reference to FIG. 12, the baseline performance of Curve A relatesto coil assembly A, with a 20° to 340° ric-rac major axes segmentorientation and no fins. Curves B to F above Curve A indicate that atthe indicated internal water flow rate along the X-axis, such curveshave a better thermal performance than the baseline performance, withincreasingly better thermal performance from Curve B to Curve F.

Test ID “G” and “H” with a 20°-340° ric-rac major axes orientation,respective fin spacing of 1.5 and 3 fins/inch (2.54 cm) and fin heightof 0.5 inch (1.27 cm) (outside the fin height parameter of the presentinvention) had consistently lower thermal performance (MBH) as indicatedby Curves G and H, respectively.

In general, the test results show that an orientation of the major axesof the generally elliptical finned segments in a generally verticaldirection) (0°) provides better thermal performance than a ric-racorientation of the major axes for tubes having the same fin height andfin spacing. Nevertheless arranging the major segments in a ric-racorientation still provides a very considerable increase in thermalperformance of a coil assembly having all of the other parameters withinthe scope of the present invention. For tubes having the same angle oforientation, namely a ric-rac or generally vertical orientation of thegenerally elliptical segments, fins having a height of 0.3125 inch(0.794 cm) provided the better thermal performance. For tubes having thesame orientation angle of their major axes and fin height, less spacingwithin the parameters of the present invention provide better thermalperformance.

The practical effect of the results shown in FIG. 12 is that coilassemblies made using the finned tubes of the present invention, havingthe combination of factors of tube shape, orientation, arrangement andspacing, and fin spacing, height and thickness, all of which must becarefully balanced, provide a dramatic increase in thermal capacity andperformance compared to other coil assemblies having the same footprint(plan area). Thus, based on the present invention, among the otherbenefits and advantages described above, a significantly morecost-effective coil assembly can be produced by providing a smaller coilassembly that results in the same heat capacity demand. This isimportant not only for increased initial commercial sales, but also forlater more cost-effective operation of evaporative heat exchangeapparatus using the coil assemblies of the present invention. For coilassemblies of the same plan area, the graph of FIG. 12 verysignificantly shows enhanced thermal performance, for the embodimentstested and the results shown in FIG. 12, up to about an 18.3% increasein MBH, comparing the results of Curve F to the baseline Curve A, asmeasured at a rate of flow of internal process fluid (water) of 8 gpmper tube (calculated as 504−426=78/426×100=18.3%).

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. In an evaporative heat exchanger comprising a plenum having a generally vertical longitudinal axis, a distributor for distributing an external heat exchange liquid into the plenum, an air mover for causing air to flow in a direction through the plenum in a direction generally countercurrent to, generally parallel to, or generally across the longitudinal axis of the plenum, and a coil assembly having a major plane and being mounted within the plenum such that the major plane is generally normal to the longitudinal axis of the plenum and such that the external heat exchange liquid flows externally through the coil assembly in a generally vertical flow direction, wherein the coil assembly comprises inlet and outlet manifolds and a plurality of tubes connecting the manifolds, the tubes extending in a direction generally horizontally and having a longitudinal axis and a generally elliptical cross-sectional shape having a major axis and a minor axis where the average of the major axis length and the minor axis length is a nominal tube outside diameter, the tubes being arranged in the coil assembly such that adjacent tubes are generally vertically spaced from each other within planes generally parallel to the major plane, the adjacent tubes in the planes generally parallel to the major plane being staggered and spaced with respect to each other generally vertically to form a plurality of staggered generally horizontal levels in which every other tube is aligned in the same generally horizontal level generally parallel to the major plane, and wherein the tubes are spaced from each other generally horizontally and generally normal to the longitudinal axis of the tube, the improvement comprising the tubes having external fins formed on an outer surface of the tubes, wherein the fins have a spacing of 1.5 to 3.5 fins per inch (2.54 cm) along the longitudinal axis of the tubes, the fins having a height extending from the outer surface of the tubes a distance of substantially 23.8% to substantially 36% of the nominal tube outside diameter, the fins having a thickness of substantially 0.007 inch (0.018 cm) to substantially 0.020 inch (0.051 cm), the tubes having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the tubes of substantially 100% to substantially 131% of the nominal tube outside diameter, and the horizontally adjacent tubes having a generally vertical center-to-center spacing of substantially 110% to substantially 300% of the nominal tube outside diameter.
 2. In an evaporative heat exchanger according to claim 1, the improvement further comprising the fins having a spacing of substantially 2.75 to substantially 3.25 fins per inch (2.54 cm) along the longitudinal axis of the tubes.
 3. In an evaporative heat exchanger according to claim 2, the improvement further comprising the fins having a spacing of substantially 3 fins per inch (2.54 cm) along the longitudinal axis of the tubes.
 4. In an evaporative heat exchanger according to claim 1, the improvement further comprising the tubes having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the tubes of substantially 106% to substantially 118% of the nominal tube outside diameter.
 5. In an evaporative heat exchanger according to claim 4, the improvement further comprising the tubes having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the tubes of substantially 112% of the nominal tube outside diameter.
 6. In an evaporative heat exchanger according to claim 1, the improvement further comprising the tubes having a generally vertical center-to-center spacing of substantially 150% to substantially 205% of the nominal tube outside diameter.
 7. In an evaporative heat exchanger according to claim 6, the improvement further comprising the tubes having a generally vertical center-to-center spacing of substantially 179% of the nominal tube outside diameter.
 8. In an evaporative heat exchanger according to claim 1, the improvement further comprising the nominal tube outside diameter being substantially 1.05 inches (2.67 cm).
 9. In an evaporative heat exchanger according to claim 1, the improvement further comprising the fins having a spacing of substantially 2.75 to substantially 3.25 fins per inch (2.54 cm) along the longitudinal axis of the tubes, the fins having a height of substantially 28% to substantially 33% of the nominal tube outside diameter, the fins having a thickness of substantially 0.009 inch (0.023 cm) to substantially 0.015 inch (0.038 cm), the tubes having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the tubes of substantially 106% to substantially 118% of the nominal tube outside diameter, and the tubes having a generally vertical center-to-center spacing of substantially 150% to substantially 205% of the nominal tube outside diameter.
 10. In an evaporative heat exchanger according to claim 9, the improvement further comprising the nominal tube outside diameter being substantially 1.05 inches (2.67 cm).
 11. In an evaporative heat exchanger according to claim 1, the improvement further comprising the fins having a spacing of substantially 3 fins per inch (2.54 cm) along the longitudinal axis of the tubes, the fins having a height of substantially 29.76% of the nominal tube outside diameter, the fins having a thickness of substantially 0.01 inch (0.025 cm) to substantially 0.013 inch (0.033 cm), the tubes having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the tubes of about 112% of the nominal tube outside diameter, and the tubes having a generally vertical center-to-center spacing of about 179% of the nominal tube outside diameter.
 12. In an evaporative heat exchanger according to claim 11, the improvement further comprising the nominal tube outside diameter being substantially 1.05 inches (2.67 cm).
 13. In an evaporative heat exchanger according to claim 1, the improvement further comprising the nominal tube outside diameter being substantially 1.05 inches (2.67 cm), the fins having a center-to-center spacing of substantially 0.286 inch (0.726 cm) to substantially 0.667 inch (1.694 cm), the fins having a height of substantially 0.25 inch (0.635 cm) to substantially 0.375 inch (0.953 cm), the tubes having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the tubes of substantially 1.05 inches (2.67 cm) to substantially 1.38 inches (3.51 cm), and the horizontally adjacent tubes having a generally vertical center-to-center spacing of substantially 1.15 inches (2.92 cm) to substantially 3.15 inches (8.00 cm).
 14. In an evaporative heat exchanger according to claim 13, the improvement further comprising the fins having a center-to-center spacing of substantially 0.308 inch (0.782 cm) to substantially 0.364 inch (0.925 cm), a height of substantially 0.294 inch (0.747 cm) to substantially 0.347 inch (0.881 cm), the fins having a thickness of substantially 0.009 inch (0.023 cm) to substantially 0.015 inch (0.038 cm), and the horizontally adjacent tubes having a generally vertical center-to-center spacing of substantially 1.57 inches (3.99 cm) to about 2.15 inches (5.46 cm).
 15. In an evaporative heat exchanger according to claim 14, the improvement further comprising the fins having a center-to-center spacing of substantially 0.333 inch (0.846 cm), a height of substantially 0.3125 inch (0.794 cm), a thickness of substantially 0.01 inch (0.025 cm) to substantially 0.013 inch (0.033 cm), the tubes having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the segments of substantially 1.175 inches (2.985 cm), and the tubes having a generally vertical center-to-center spacing of substantially 1.88 inches (4.78 cm).
 16. In an evaporative heat exchanger according to claim 1, the improvement further comprising the major axes of the tubes being generally parallel to the longitudinal axis of the plenum.
 17. In an evaporative heat exchanger according to claim 1, the improvement further comprising the major axes of the tubes being angled with respect to the longitudinal axis of the plenum.
 18. In an evaporative heat exchanger according to claim 17, the improvement further comprising the major axes of the tubes of adjacent tubes on different vertical levels being angled in opposite directions with respect to each other and to the longitudinal axis of the plenum.
 19. In an evaporative heat exchanger according to claim 18, the improvement further comprising the angle of the major axes of the tubes on a first generally horizontal level being greater than 0° to about 25° degrees from the longitudinal axis of the plenum and the angle of the major axes of the tubes on the next vertically adjacent generally horizontal level being about 335° to less than 360° from the longitudinal axis of the plenum.
 20. In an evaporative heat exchanger according to claim 19, the improvement further comprising the angle of the major axes of the tubes on a first generally horizontal level being about 20 degrees from the plane of the return bends and the angle of the major axes of the tubes on the next vertically adjacent generally horizontal level being about 340 degrees from the longitudinal axis of the plenum.
 21. In an evaporative heat exchanger according to claim 1, the improvement further comprising the fins having undulations in and out of a plane of material used to make the fins.
 22. In an evaporative heat exchanger according to claim 1, wherein the finned tubes are galvanized such that the fins after galvanization are thicker at a base proximal to the outer surface of the tube than at a tip of the fins distal from the outer surface of the tube.
 23. In an evaporative heat exchanger according to claim 1, wherein the tubes are serpentine tubes having a plurality of segments and a plurality of return bends, the return bends being oriented in generally vertical planes, the segments of each tube connecting the return bends of each tube and extending between the return bends in a direction generally horizontally, the segments having a longitudinal axis and a generally elliptical cross-sectional shape having a major axis and a minor axis where the average of the major axis length and the minor axis length is a nominal tube outside diameter, the segments being arranged in the coil assembly such that the segments of adjacent tubes are generally vertically spaced from each other within planes generally parallel to the major plane, the segments of adjacent tubes in the planes generally parallel to the major plane being staggered and spaced with respect to each other generally vertically to form a plurality of staggered generally horizontal levels in which every other segment is aligned in the same generally horizontal level generally parallel to the major plane, and wherein the segments are spaced from each other generally horizontally and generally normal to the longitudinal axis of the segment connected to the return bend, the improvement comprising the segments having external fins formed on an outer surface of the tubes, wherein the fins have a spacing of substantially 1.5 to substantially 3.5 fins per inch (2.54 cm) along the longitudinal axis of the segments, the fins having a height extending from the outer surface of the segments a distance of substantially 23.8% to substantially 36% of the nominal tube outside diameter, the fins having a thickness of substantially 0.007 inch (0.018 cm) to substantially 0.020 inch (0.051 cm), the segments having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the segments of substantially 100% to substantially 131% of the nominal tube outside diameter, and the horizontally adjacent segments having a generally vertical center-to-center spacing of substantially 110% to substantially 300% of the nominal tube outside diameter.
 24. In an evaporative heat exchanger according to claim 23, the improvement further comprising the fins having a spacing of substantially 2.75 to substantially 3.25 fins per inch (2.54 cm) along the longitudinal axis of the segments, the fins having a height of substantially 28% to substantially 33% of the nominal tube outside diameter, the fins having a thickness of substantially 0.009 inch (0.023 cm) to substantially 0.015 inch (0.038 cm), the segments having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the segments of substantially 106% to substantially 118% of the nominal tube outside diameter, and the horizontally adjacent segments having a generally vertical center-to-center spacing of substantially 150% to substantially 205% of the nominal tube outside diameter.
 25. In an evaporative heat exchanger according to claim 24, the improvement further comprising the fins having a spacing of substantially 3 fins per inch (2.54 cm) along the longitudinal axis of the segments, the fins having a height of substantially 29.76% of the nominal tube outside diameter, the fins having a thickness of substantially 0.01 inch (0.025 cm) to substantially 0.013 inch (0.033 cm), the segments having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the segments of substantially 112% of the nominal tube outside diameter, and the horizontally adjacent segments having a generally vertical center-to-center spacing of substantially 179% of the nominal tube outside diameter.
 26. In an evaporative heat exchanger according to claim 23, the improvement further comprising the return bends having a circular cross-section with an outside diameter of substantially 1.05 inches (2.67 cm) and the nominal tube outside diameter being substantially 1.05 inches (2.67 cm).
 27. In an evaporative heat exchanger according to claim 23, the improvement further comprising the return bends having a generally elliptical cross-section and the nominal tube outside diameter being substantially 1.05 inches (2.67 cm).
 28. In an evaporative heat exchanger according to claim 23, the improvement further comprising the major axes of the segments being generally parallel to the plane of the return bends.
 29. In an evaporative heat exchanger according to claim 23, the improvement further comprising the major axes of the segments being angled with respect to the plane of the return bends.
 30. In an evaporative heat exchanger according to claim 29, the improvement further comprising the major axes of the segments of adjacent tubes on different vertical levels being angled in opposite directions with respect to each other and to the plane of the return bends.
 31. In an evaporative heat exchanger according to claim 30, the improvement further comprising the angle of the major axes of the segments on a first generally horizontal level being greater than 0° to about 25° degrees from the plane of the return bends and the angle of the major axes of the segments on the next vertically adjacent generally horizontal level being about 335° to less than 360° from the plane of the return bends.
 32. In an evaporative heat exchanger according to claim 31, the improvement further comprising the angle of the major axes of the segments on a first generally horizontal level being about 20 degrees from the plane of the return bends and the angle of the major axes of the segments on the next vertically adjacent generally horizontal level being about 340 degrees from the plane of the return bends.
 33. In an evaporative heat exchanger according to claim 31, the improvement further comprising the fins having a spacing of substantially 2.75 to substantially 3.25 fins per inch (2.54 cm) along the longitudinal axis of the segments, the fins having a height of substantially 28% to substantially 33% of the nominal tube outside diameter, the fins having a thickness of substantially 0.009 inch (0.023 cm) to substantially 0.015 inch (0.038 cm), the segments having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the segments of substantially 106% to substantially 118% of the nominal tube outside diameter, and the horizontally adjacent segments having a generally vertical center-to-center spacing of substantially 150% to substantially 205% of the nominal tube outside diameter.
 34. In an evaporative heat exchanger according to claim 31, the improvement further comprising the fins having a spacing of substantially 3 fins per inch (2.54 cm) along the longitudinal axis of the segments, the fins having a height of substantially 29.76% of the nominal tube outside diameter, the fins having a thickness of substantially 0.01 inch (0.025 cm) to substantially 0.013 inch (0.033 cm), the segments having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the segments of substantially 112% of the nominal tube outside diameter, and the segments having a generally vertical center-to-center spacing of substantially 179% of the nominal tube outside diameter.
 35. In an evaporative heat exchanger comprising a plenum having a generally vertical longitudinal axis, a distributor for distributing an external heat exchange liquid into the plenum, an air mover for causing air to flow in a direction through the plenum in a direction generally countercurrent to, generally parallel to, or generally across the longitudinal axis of the plenum, and a coil assembly having a major plane and being mounted within the plenum such that the major plane is generally normal to the longitudinal axis of the plenum and such that the external heat exchange liquid flows externally through the coil assembly in a generally vertical flow direction, wherein the coil assembly comprises inlet and outlet manifolds and a plurality of tubes connecting the manifolds, the tubes extending in a direction generally horizontally and having a longitudinal axis and a generally elliptical cross-sectional shape having a major axis and a minor axis where the average of the major axis length and the minor axis length is a nominal tube outside diameter, the tubes being arranged in the coil assembly such that adjacent tubes are generally vertically spaced from each other within planes generally parallel to the major plane, the adjacent tubes in the planes generally parallel to the major plane being staggered and spaced with respect to each other generally vertically to form a plurality of staggered generally horizontal levels in which every other tube is aligned in the same generally horizontal level generally parallel to the major plane, and wherein the tubes are spaced from each other generally horizontally and generally normal to the longitudinal axis of the tube, the improvement comprising at least one of the tubes being a finned tube having external fins formed on an outer surface of the tubes, wherein the fins have a spacing of 1.5 to 3.5 fins per inch (2.54 cm) along the longitudinal axis of the tubes, the fins having a height extending from the outer surface of the tubes a distance of substantially 23.8% to substantially 36% of the nominal tube outside diameter, the fins having a thickness of substantially 0.007 inch (0.018 cm) to substantially 0.020 inch (0.051 cm), the tubes having a center-to-center spacing generally horizontally and generally normal to the longitudinal axis of the tubes of substantially 100% to substantially 131% of the nominal tube outside diameter, and the horizontally adjacent tubes having a generally vertical center-to-center spacing of substantially 110% to substantially 300% of the nominal tube outside diameter.
 36. In an evaporative heat exchanger according to claim 35, the improvement further comprising a plurality of the tubes in the coil assembly being the finned tubes.
 37. In an evaporative heat exchanger according to claim 36, the improvement further comprising a majority of the tubes in the coil assembly being the finned tubes. 